1. Field of the Invention
The present invention relates to an X-ray mask for use in X-ray lithography in which a pattern is transferred to a substrate by the radiation of X-rays, and an exposure method employing the same.
2. Description of the Related Art
Generally, X-ray lithography technology involves a process of transferring very fine patterns of about 0.3 .mu.m or less to a substrate.
FIG. 1 is a plan view of a conventional X-ray mask employed to transfer the minute patterns to a substrate, and FIGS. 2 and 3 are sections taken along the line II--II of FIG. 1, respectively showing a state in which a supporting frame to be described later has not yet been etched and a state in which it has already been etched. An X-ray mask 2 shown in these figures has a membrane 4 made of a material, such as silicon nitride, which is highly transparent to X-rays, a mask pattern 6 made of a material, such as tungsten, which absorbs X-rays, the mask pattern 6 being formed on the upper surface of the membrane 4, a supporting frame 8 of Si formed on the lower surface of the membrane 4 for mechanically supporting the membrane 4, and an etching mask 10 provided on the lower surface of the supporting frame 8 to be used as a mask during etching of the supporting frame 8.
In the thus-arranged conventional X-ray mask structure, the membrane 4 and the mask pattern 6 are sequentially formed as very thin films having a thickness of several .mu.m by vacuum deposition, sputtering, or the CVD method. In consequence, when the central portion of the disk-shaped supporting frame 8 is etched after the membrane 4 and the mask pattern 6 have been formed on the upper surface of the supporting frame 8 and after the etching mask 10 has been formed on the lower surface thereof, as shown in FIG. 2, the thin membrane 4 is deformed due to the reduction in the internal stress in the membrane 4 and in the mask pattern 6. As shown in FIG. 3, the mask pattern 6 formed on the upper surface of the membrane 4 is shifted from its predetermined correct position, thereby causing a distortion of the pattern.
Further, the supporting frame 8, the membrane 4, and the mask pattern 6 may be thermally distorted, respectively, because of changes in an ambient temperature of the location where the X-ray mask 2 is placed, which may also lead to pattern distortion.
FIG. 4 shows a mask pattern 6 in which such a pattern distortion has been generated. In FIG. 4, the outline of the mask pattern 6 as viewed from above is schematically represented by the square, which is divided into 25 pattern portions. The pattern portions A.sub.1, A.sub.2, A.sub.3 shown by the broken line represent those located at their correct positions, whereas the pattern portions B.sub.1, B.sub.2, B.sub.3 . . . shown by the solid line are those which have been displaced from their correct positions by pattern distortion. In particular, the hatched areas represent the mask pattern 6, which has been shifted from its correct position by pattern distortion.
If an X-ray mask 2 with the mask pattern 6 which has been displaced from its correct position is employed to transfer the circuit patterns to the substrate, the accuracy with which the circuit patterns are aligned with the substrate reduced to a great extent, and this makes formation of the very fine patterns on the substrate with a high degree of accuracy difficult.
In order to obviate this problem, Japanese Patent Laid-Open No. 62-122216 discloses an X-ray mask in which electrostriction or magnetostriction elements bonded to a supporting frame by an adhesive are deformed so as to correct the distortion in the mask. However, this X-ray mask structure suffers from a problem in that the adhesive is readily removed due to repeated vibrations of these electrostriction or magnetostriction elements. This in turn causes removal of the elements from the supporting frame, which leads to a reduction in the reliability of the mechanical structure of the mask. The X-ray mask also has a disadvantage in that it takes much time and trouble for the elements to be bonded to the supporting frame.